Optoelectronic symptoms, and in particular laser scanners, are suitable for distance measurements which require a large horizontal angular range of the measurement system. In a laser scanner, a light beam generated by a laser periodically sweeps over a monitored zone with the help of a deflection unit. The light is remitted at objects in the monitored zone and is evaluated in the scanner. A conclusion is drawn on the angular location of the object from the angular position of the deflection unit and additionally on the distance of the object from the laser scanner from the time of flight of light while using the speed of light. Two general principles are known for determining the time of flight of light for conventional light scanners. In phase-based processes, the continuous transmitted light is modulated and the phase shift of the received light with respect to the transmitted light is evaluated. In pulse-based processes or pulse transit time processes, the transmitter works in single pulse operation at comparatively high pulse energies and the laser scanner measures object distances with reference to the time of flight between the transmission and reception of a single light pulse. In a pulse averaging process known from EP 2 469 296 B1, a plurality of individual pulses are transmitted for a measurement and the received pulses are statistically evaluated.
The location of an object in the monitored zone is detected in two-dimensional polar coordinates using the angular data and the distance data. The positions of objects can thus be determined or their contour can be determined. The third spatial coordinate can likewise be detected by a relative movement in the transverse direction, for example by a further degree of freedom of movement of the deflection unit in the laser scanner or in that the object is conveyed relative to the laser scanner. Three-dimensional contours can thus also be measured.
In addition to such measurement applications, laser scanners are also used in safety technology for monitoring a danger source, such as a dangerous machine. Such a safety laser scanner is known from DE 43 40 756 A1. In this process, a protected field is monitored which may not be entered by operators during the operation of the machine. If the laser scanner recognizes an unauthorized intrusion into the protected field, for instance a leg of an operator, it triggers an emergency stop of the machine. Other intrusions into the protected field, for example by static machine parts, can be taught as permitted in advance. Warning fields are frequently disposed in front of the protected fields where intrusions initially only result in a warning to prevent the intrusion into the protected field and thus the securing in good time and so increase the availability of the plant. Safety laser scanners usually work as pulse-based.
Safety laser scanners have to work particularly reliably and must therefore satisfy high safety demands, for example the EN13849 standard for safety of machinery and the machinery standard EN1496 for electrosensitive protective equipment (ESPE). To satisfy these safety standards, a series of measures have to be taken such as a safe electronic evaluation by redundant, diverse electronics, functional monitoring or special monitoring of the contamination of optical components, in particular of a front screen.
One of these measures which turn a laser scanner into a safety laser scanner is the use of an internal reference target system with whose aid the error-free function of the distance measurement and its unimpaired sensitivity are checked. FIG. 5 shows such a conventional safety laser scanner 100 with a reference target 102 in a schematic plan view. This reference target 102 is optically scanned in every revolution of the deflection unit and the signal echo is evaluated with respect to signal strength and distance value. The comparison of this measurement with a taught expected value allows the assessment of whether the detection capability of the safety laser scanner is limited. In addition, a current distance offset which is caused by temperature-dependent internal signal transit time fluctuations can be determined and corrected from this measurement.
This monitoring and correction function can only be perceived when the reference target signal is independent of external circumstances such as external light, contamination and background emissions. This delineation from environmental influences is achieved in the prior art in that a contiguous angular range 104 of the viewing range is used exclusively for the reference target measurement. The carrier at which the deflection unit with motor, rotating mirror and angular encoder is suspended is also located in the angular range 104. The reference target 102 is also fastened to the carrier.
The advantage of the conventional implementation is the possibility of a continuous system testing with the aid of the actual measurement system. The disadvantage of this design is the reduction of the usable viewing range because the carrier with the reference target 104 blocks the optical beam path to the outside. The viewing range 106 is thereby restricted to approximately 270°. Applications in which a larger viewing range 106 is to be monitored can therefore only be covered by at least two safety laser scanners. In this respect, the situation shown in FIG. 5 with a dead zone in the angular range 104 is even shown too optimistically. The more compact the construction of the safety laser scanner, the more difficult it becomes to realize such a narrow angular range 104 as a dead zone or to achieve only a reduction by only approximately 90° at all.
A laser scanner is known from EP 2 482 094 B1 in which the test target or reference target does not lie in the scan plane, with the scan beam being conducted from the scan plane to the test target with the aid of a deflection unit. In an embodiment, the inner surface of a curved front screen forms the deflection unit. The deflection unit, however, shadows the angular range of the test goal in exactly the same way as the above-described carrier. In the case of the embodiment in which the front screen forms the deflection unit, it is separately mirrored in this angular range. This arrangement thus does not solve the problem of the dead zone.